metal lowbit kernels: qmv_fast optimization

[manuelcandales]

This PR does the following modifications:

• Previously, the metal lowbit kernels consumed the scales and zeros transposed, (i.e. num_groups x N). This PR changes that, so that now, the scales and zeros are consumed in the same shape as produced by the _quantize function, (i.e. N x num_groups)
• Packing for 3, 5, 6, & 7 bit is changed. The bits are stored contiguously now, jumping to the next byte when we reach the end of the current byte.
• qmv_fast optimization from MLX is adapted and expanded to support 1, 5 & 7 bits (not currently supported in MLX)

[pytorch-bot]

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